Numerous proposals are known from the prior art to provide olefin polymerization catalysts by combining a solid component comprising at least magnesium, titanium and chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
Supported coordination catalysts of this type are disclosed in numerous patents. The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components in a dashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active catalyst systems, practical considerations have led the workers in the art to concentrate on certain preferred components. The procatalysts typically comprise magnesium chloride, titanium chloride, generally in tetravalent form, and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic ester such as ethyl-paramethoxybenzoate(ethylanisate) or methyl-p-toluate.
While the selection of cocatalyst and selectivity control agent affects the performance of those catalyst systems, the component which appears to be subject to most significant improvement with respect to activity and productivity of the system is the procatalyst.
Preferred methods of preparing such procatalysts are claimed in U.S. Pat. Nos. 4,329,253; 4,393,182; 4,400,302; and 4,414,132. These procatalysts are highly active and stereospecific. The typical manner of preparing such procatalysts involves the reaction of the magnesium compound, titanium tetrachloride and electron donor. The resulting solid particles are then contacted with additional quantities of TiCl.sub.4 and are completed by washing off excess TiCl.sub.4 using light hydrocarbons (e.g., isooctane and isopentane) and drying.
As part of the typical process described above starting with magnesium alkoxides and TiClhd 4, contaminants of the formula TiCl.sub.3 OR are formed. While it would in principle be possible to remove these compounds by fractional distillation of the liquid phase, this method is however not adequately energy-efficient and in cases where the contaminants have a boiling point close to that of TiCl.sub.4, this being so for TiCl.sub.3 OC.sub.2 H.sub.5, the distillation route requires elaborate equipment and is not easy to implement in industrial plants.
A simpler method is therefore desirable and the technical problem underlying this invention is to achieve this.